1. Field of The Invention
Applicant""s invention relates to a solvent based adhesive at high solids in a solvent blend affording less than 250 grams per liter VOC content while maintaining exceptional contact bond adhesive properties.
2. Background Information
Presently, in the high pressure laminate (HPL) industry, solvent based or water borne contact bond adhesives are used to adhere a wide range of substrates, including wood in the form of particle board and plywood, plastics such as fiberglass, acrylonitrile-butadiene-styrene (ABS), metals such as steel, iron, aluminum, and the like. The adhesive is applied to both substrates using either a spraying or brushing technique. After the adhesive is applied, the coated substrates are allowed to dry at room temperature or dried on a conveyor in an oven to accelerate the removal of residual traces of solvent. This technique can be applied to post-forming countertops, cabinet, and furniture applications if needed. The substrates are then bonded with pressure to insure proper surface contact.
The contact bond adhesives are based on the polychloroprene polymer for its excellent properties imparted to the adhesive. The properties affected are rapid green strength development, aggressive grab tack, surface adhesion, open tack time, viscosity, spray characteristics, and bond strength development. Apart from the polychloroprene, phenolic resins and tackifiers added to the polychloroprene also play a major role in enhancing the adhesive properties synergistically. The phenolic resins when complexed with magnesium oxide increase heat resistance, bond strength and substrate adhesion of the adhesive. Resins such as rosin esters, polyterpenes, terpene phenolics, aromatic hydrocarbons, metallic resinates, and coumarone-indene resin affect bondability, tack and cohesive strength.
Metallic oxides such as magnesium oxide and zinc oxide are used. The magnesium oxide provides three advantages. First, it reacts with the phenolic resin to form a complex. Second, it functions as a curative. And third it acts as an acid acceptor thereby protecting sensitive substrates against released hydrochloric acid. The zinc oxide functions as a curative and as an acid acceptor.
Another important ingredient to the polychloroprene based adhesive is antioxidants such as hindered substituted phenols. These antioxidants protect the adhesive film from degrading on long-term aging of the bonded substrates.
The carrier or solvent medium for the solid ingredients is a blend of solvents typically aliphatics, ketones, esters, and aromatics. The optimum solvent blend is very important to the adhesive composition because the solvents influence polymer solubility, viscosity, spray characteristics, phasing, shelf life, open tack time, strength development, and evaporation time. It is desired to have a solvent blend that not only provides optimal characteristics to the adhesive, but also maintains a lower volatile organic compound content. At present only a few solvents are considered VOC exempt such as acetone, methyl acetate, and methylene chloride . However, both acetone and methyl acetate are poor solvents for polychloroprene in contact bond adhesives. Furthermore, methylene chloride is a restricted solvent due to its possible carcinogenic effect. Recently OSHA reduced the permissible exposure limit (PEL) for methylene chloride to 25 ppm.
Typically the HPL industry and other related industries use solvent based contact bond adhesives having a VOC content greater than 400 grams per liter. Recently, the California Air Quality District reduced the allowable limit of VOCs from 540 to 400 grams per liter and further reduced the maximum level to 250 grams per liter. No contact bond adhesive is known in the industry that has a VOC content as low as 250 grams per liter. In order to satisfy this need in the industry, the present adhesive was developed. Surprisingly, this adhesive exhibits exceptional contact bond adhesive characteristics without the regulatory disadvantages normally associated with high VOC content contact adhesives.
Evaluation and comparative testing were made of the present invention, designated T245 for the comparison study, with C2725, a typical extremely flammable polychloroprene adhesive found on the market today in the United States excluding some areas of California. The technical data for both compounds is as follows:
From these studies it was found that C2725 had higher bond strengths than T245, particularly at elevated temperatures. However, T245 did exhibit a greater increase in bond strength at lower temperatures over time compared to C2725 which is significant considering the total VOC content of T245 is less than half that of C2725.
The primary object of the present invention was to provide a novel solvent based polychloroprene contact bond adhesive having a volatile organic compound content less than 250 grams per liter at a high solids content while maintaining exceptional contact bond adhesive properties. The adhesive contains a low viscosity fast crystallizing polychloroprene polymer, a gel polychloroprene polymer, a heat reactive phenolic resin, an aromatic hydrocarbon plasticizer, a terpene phenolic resin, an aromatic hydrocarbon resin, a zinc resinate, an antioxidant, an alkaline earth metal oxide, and water in a blend of solvents including hexane, acetone, methyl acetate, and toluene. The composition of the present invention is useful to a practitioner since it provides for a VOC level less than 250 grams per liter according to the guidelines set forth by California Air Districts EPA Method 24 and provides exceptional contact bond adhesive properties.